The present invention relates to an electroconductive silicone rubber composition, or, more particularly, to an electroconductive silicone rubber composition which can be prepared industrially at a relatively low cost, has excellent workability and is capable of giving a cured silicone rubber article having good rubbery properties despite the use of a substantial amount of a metallic electroconductivity-imparting agent to be useful as a material of various kinds of electrodes, electroconductive rubber rollers, shielding members against electromagnetic waves, electronic devices and the like. The invention also relates to a cured electroconductive silicone rubber article obtained by shaping and curing the above mentioned electroconductive silicone rubber composition.
As is well known, silicone rubbers in general have excellent heat and cold resistance and weatherability as well as excellent electric insulation so that they are widely used in a variety of application fields as an electrically insulating rubbery material. On the other hand, electroconductive silicone rubbers are also known and have a volume resistivity of, for example, from 10.sup.-4 to 10.sup.6 ohm.multidot.cm. Such an electroconductive silicone rubber composition is prepared by compounding an insulating silicone rubber composition with a substantial amount of an electroconductivity-imparting agent depending on the electroconductivity desired. The electroconductivity-imparting agent suitable for compounding with a silicone rubber composition includes so-called .pi.-electron-transfer type electroconductive materials such as carbon black, graphite powder, carbon fibers and the like and so-called free electron-transfer type electroconductive materials such as powders, flakes, fibers and the like of a metal or metalloid, e.g., silver, nickel, copper zinc, iron, gold and silicon.
A highly electroconductive silicone rubber composition having a volume resistivity not exceeding 10.sup.-1 ohm.multidot.cm can be obtained only by using a metallic electroconductivity-imparting agent. For example, a highly electroconductive silicone rubber composition having a volume resistivity of from 10.sup.-3 to 10.sup.-4 ohm.multidot.cm can be obtained by incorporating a sufficiently large amount of a powder of a noble metal such as silver, gold and the like. A fatal problem in such a highly electroconductive silicone rubber composition is, needless to say, the very high production costs thereof due to the expensiveness of the noble metal powder.
In this regard, a proposal has been made in Japanese Patent Kokai No. 58-63198 and No. 59-100756, according to which the electroconductivity-imparting agent is a powder of an inorganic material such as glass, mica, alumina, carbon and the like of which the particles are coated or plated with a metal such as silver, nickel and the like. A problem in the electroconductive silicone rubber composition of this type is in the relatively low stability of the electroconductivity because the metallic surface on the filler particles is gradually oxidized in the lapse of time to cause an increase in the volume resistivity of the silicone rubber composition. Although admixture of an amine compound as an antioxidant is effective to some extent to prevent oxidation of the metallic surface by air, the oxidation-preventing effect thereof is lost almost completely when the silicone rubber article is kept at an elevated temperature.
It is also taught in Japanese Patent Publication No. 46-41694 that a highly electroconductive silicone rubber composition can be obtained by compounding a relatively small amount of a metallic electroconductivity-imparting agent with an insulating silicone rubber composition when the metallic powder is used in combination with a fine powder of a cured silicone rubber. According to the disclosure therein, the powder of the cured silicone rubber used is prepared by pulverizing a cured mass of a silicone rubber. A problem in the use of such a powder of a cured silicone rubber is that the silicone rubber composition compounded therewith has very poor workability in processing and molding. Further, a proposal has been made in Japanese Patent Kokai No. 61-108661 and No. 63-251464 for the preparation of an electroconductive silicone rubber composition in which the electroconductivity-imparting agent is a powder of a cured electroconductive silicone rubber containing a relatively large amount of carbon black. Although such an electroconductive silicone rubber is advantageous in respect of the stability of the electroconductivity, a problem therein is that a highly electroconductive silicone rubber composition having a volume resistivity not exceeding 10.sup.-1 ohm.multidot.cm can hardly be obtained by compounding a carbon black-loaded cured silicone rubber powder.